The present invention relates to a process and a machine for moulding composite plastic material, in particular a plastic foamed material in combination with a rigid plastic material, such as, e.g., foamed polystyrene coupled with rigid polystyrene sheet.
The term "rigid" is meant in the instant disclosure in a relative meaning, i.e., in order to describe a material showing generally higher stiffness characteristics than of the plastic material that the rigid material is coupled with; consequently, by such a term, also a thin-sheet material is understood.
In order to mould these materials, which require steam to be applied in order to cause the foamed plastic material to undergo sintering and simultaneously adhere to the rigid material, from Italian patent No. 1 185 507 the use is already known of a mould constituted by two half-moulds which define, between them, a moulding hollow suitable for containing both materials, one of which, i.e., the more rigid, material, in sheet form, is first preformed while only being in contact with the bottom half-mould, and the other material is then moulded atop the first one by means of a sintering step inside the closed mould.
According to this technique, the rigid, sheet-shaped plastic material is fastened at its extreme edges and is deformed while hot, initially due to the movement of the relevant half-mould beyond the plane on which the sheet material lays, and then the material is caused to exactly adhere to the half-mould, by creating a vacuum between said half-mould and the material.
Subsequently, the upper half-mould is lowered and inside the resulting hollow, the foamed plastic material is moulded by applying steam in a pulsed way and alternating steam application steps and vacuum steps, in order to enable the foamed material to undergo sintering also in its regions adjacent to the more rigid material, and hence create an effective adhesion of both materials to each other.
The method and equipment known from the prior art show the advantage that the moulding process can be carried out by means of one single mould consisting of two half-moulds which are mobile relatively to each other, hence they are advantageous from the constructive view point.
Unfortunately, they are unsuitable if one wishes to produce a composite material consisting of a rigid sheet on both or more mutually opposite faces of the foamed plastic material.
On the other hand, the need is more and more felt of producing composite materials consisting of sheets of more rigid materials on their external faces, both because such a structure endows the resulting article of manufacture with a higher stiffness, and because the possibilities of use and the range of obtainable articles would be considerably extended.
In fact, the external surface made of such a material supplies the end manufactured article with characteristics of impermeability, hardness and impact strength, besides a more pleasant appearance and the possibility of being washed, a property which, on the contrary, foamed polystyrene, per se, does not display. For example, at present, in packaging sector, in which foamed polystyrene is widely used owing to its properties of lightness, insulation, moulding ability and low cost, the material must be mostly discarded after use, because it is damaged and cannot be used again, and this matter of fact causes obvious disposal and pollution problems. On the contrary, the material with hardened, smooth and compact surfaces on its external faces, could be used again. In any case, many articles could be manufactured, which at present have to be produced with other materials and more expensive processes and machinery.